In recent years, attention has been drawn to rechargeable batteries with a nonaqueous electrolyte. This is considered attributable mainly to a success in the development of relatively safe negative electrode materials and to the realization of high-voltage batteries by virtue of an enhancement in decomposition voltage of nonaqueous electrolytes. Among the rechargeable batteries with a nonaqueous electrolyte, rechargeable batteries using lithium ions are promising batteries which can realize high-energy density batteries because of particularly high discharge potential. Further, in particular, putting large batteries typified by batteries for electric automobiles to practical use has recently become more and more expected. For such high-energy batteries as well, a further improvement in techniques for ensuring safety is necessary.
In order to realize high-capacitance or high-output batteries, ensuring safety upon internal shortcircuiting is important. Various experimental results, which have hitherto been carried out, show that ignition upon internal shortcircuiting possibly occurs through the following mechanism.
In the charged state, a voltage V of not less than 4 V is applied across the positive electrode and the negative electrode, and energy corresponding to the charge capacitance is stored between the positive electrode and the negative electrode. Regarding stored energy, however, the following energy should be further taken into consideration. The positive electrode current collector and the negative electrode current collector have a large area S and face each other while leaving a short distance d therebetween. A high-permittivity ε material (active material and electrolyte-impregnated separator) is inserted between the positive electrode current collector and the negative electrode current collector. Therefore, when both sides (top surface and back surface) are taken into consideration, energy E (=(½)×C×V2) as a capacitor with a capacitance C (=ε=2×S/d) is included. When internal shortcircuiting occurs in this state, before the release of energy of the active material, electrons having high mobility accumulated within the current collector first flow and energy as a capacitor is released in a very short time. This energy density is generally considered to be higher than energy density involved in subsequent discharge of the active material.